in the early 1960's, they became the first roles monument. the way city spread changed with the invention of the cable car. >> people know in san francisco, first thing they think about is, let's go across america, cities and towns, homes and businesses all depend upon one basic resource. modern civilization and life itself would be impossible without it. woman: okay, so today, we're going to look at how do we get our water? narrator: and today, it's a matter of simply turning on the tap. so often, we forget about the value of water. water is a commodity that is essential to life. 100 years ago, it would have been hard to imagine turning on the tap water. and now, it's an expectation. narrator: over 300 million people live in the united states. and each person uses an average of 100 gallons of water every day. man: what it takes to actually make clean water is somewhat a mystery to most customers. woman: so how does water get from the river into your house, or here at school? woman: somebody has to bring that water to us, and somebody has to take it away when we're fini